The tendencies of iron or steel surfaces to corrode is well known. Zinc is one of the most widely used metallic coatings applied to steel surfaces to protect them from corrosion. In the past, the principal methods of applying such coatings were hot-dipping, also known as galvanizing and the electroplating of a zinc layer onto the steel. Zinc has been electroplated on the steel surfaces from various plating baths, preferably from acid plating baths, for providing protection of steel surfaces for various uses.
It has been known as in the U.S. Pat. No. 2,419,231 to improve the corrosion resistance of the coating layer by using for the coating an alloy high in zinc and low in nickel. This alloy is co-deposited from the electrolytic plating bath onto the steel substrate. Continuous steel strip, alloy-plated in accordance with the teachings of the patent, when subjected to forming and finishing operations, tends to form cracks in the coating because of the brittleness of the alloy. However, subsequent improvements, as in U.S. Pat. No. 3,420,754 teaching an improvement in corrosion resistance by a slight increase in the nickel content of the deposited alloy, have been forthcoming. Moreover, improvements in electroplate uniformity and further corrosion improvement by nickel priming have been accomplished as disclosed in U.S. Pat. No. 4,282,073. Also, the alloy plating may be further coated with a metallic zinc sacrificial layer, as disclosed in U.S. Pat. No. 4,314,893.
In some cases with substrates protected with zinc-plated layers it has been proposed to subsequently treat the surface with a chromate conversion coating or passivating film, as has been shown in Japanese Patent Disclosure No.: Showa 57-174469, or U.S. Pat. No. 4,216,272. Or to treat the zinc with a chrome ester cover layer such as taught in Japanese Patent Disclosure No.: Showa 56-113387.
It has also been known to protect steel surfaces against corrosion by using coating compositions that contain a hexavalent-chromium-providing substance as well as further containing a finely divided metal. For example, U.S. Pat. No. 3,687,739 discloses the preparation of a treated metal surface wherein such treatment includes application of a composition containing, among other constituents but as critical ingredients, chromic acid and a particulate metal. As has been disclosed in U.S. Pat. No. 3,671,331 the metals of the substrate for protection are advantageously metals from copper through zinc, inclusive, on the electromotive force series, as well as alloys of such metals wherein such metals are present in major amount. After the chromium containing bonding compositions are applied to such metal substrate, they are most always topcoated with a weldable primer topcoat composition. Such topcoats may then be cured by elevated temperature baking. It has also been known to coat zinc plated steel, typically in sheet form, with weldable zinc rich primers. Thus, in U.S. Pat. No. 4,079,163 it is shown to coat weldable primer over chromate treated galvanized steel.
It would however be further desirable to protect ferrous metals in corrosive environments, such as presented by metal composites wherein dissimilar metallic surfaces are in contact, by extending even further the corrosion resistance by coating technique prior to composite formation. It would be also desirable to provide the resulting coated article with a wide variety of worthwhile characteristics. Exemplary of these characteristics before composite formation would be coating adhesion during metal forming operation. During composite formation, such characteristics would include retention of weldability where the coated substrate would otherwise be weldable. It would be well to be able to provide coating procedures tailored to fast, economical operations, especially for the coating of steel in coil form, so as to provide an eventual composite product useful for example in the automotive industry.